A current challenge for optical communications is to expand the capacities of optical backbone network to cope with the possible future explosive expansion of information communications traffic. Various approaches are being taken to the challenge. One of the approaches is to carry out research and development regarding an improvement in usage efficiency of an optical frequency band.
In optical networks, optical frequency bands are used in accordance with the Dense Wavelength Division Multiplexing (DWDM) system standardized by the Telecommunication Standardization sector of the International Telecommunication Union (ITU-T). In the DWDM system, the entire available optical frequency band is divided into narrow segments by a grid with constant width, called a wavelength grid, and optical signals in one wavelength channel are allocated within a grid spacing (ITU-T recommendation G.694.1).
In a flexible frequency grid that is standardized by ITU-T recommendation G.694.1, the minimum channel spacing is set at 12.5 GHz instead of 50 GHz used conventionally, and a frequency slot width is variable by 12.5 GHz. This makes it possible to allocate a frequency slot of different widths to each optical path; accordingly, it becomes possible to minimize an optical bandwidth to be allocated to an optical path.
That is to say, the flexible frequency grid enables to allocate an optical bandwidth only as needed. Specifically, for example, it is only necessary in the flexible frequency grid to allocate an optical bandwidth of 12.5 GHz if the required optical bandwidth is equal to 12.5 GHz and to allocate an optical bandwidth of 50 GHz if it is equal to 50 GHz. In contrast, in a fixed grid before the introduction of the flexible frequency grid, if the frequency slot width is set at 50 GHz, an optical bandwidth of 50 GHz is allocated equally to each optical path regardless of a required optical bandwidth. Even though a required optical bandwidth is 12.5 GHz, for example, the optical bandwidth to be allocated is 50 GHz; accordingly, a bandwidth by 37.5 GHz is allocated in vain. In contrast, the flexible frequency grid makes it possible to reduce such unnecessary allocation of the bandwidth, so it enables the optical frequency band usage efficiency to improve.
However, even though the flexible frequency grid is used, an unused frequency region can arise, and a fragmentation of the optical bandwidth allocation may arise. It is considered that an optical path with four slots in width is intended to be generated and there are ten empty slots as a whole in the optical frequency band of an optical fiber, for example. If the ten empty slots are composed of five pairs of empty slots each of which includes two consecutive slots, it is impossible to generate an optical path with four slots in width. That is to say, despite the fact that there are sufficient empty slots in total, it is impossible to secure consecutive empty slots because the respective empty slots are disposed in fragments. As a result, the situation may occur where it is impossible to allocate to an optical path a wide optical bandwidth with which high-capacity or long-distance communications can be achieved. This is called a fragmentation of an optical frequency, which is made easier to arise as the center optical frequency of the optical path or the number of slots of the optical bandwidth is changed more repeatedly.
Patent Literature 1 discloses a technology to solve the above-mentioned problem that the fragmentation of the optical frequency arises.
In a method for eliminating the fragmentation of an optical spectrum in an optical network described in Patent Literature 1, first, optical signals are allocated to a plurality of frequency slots. This allocation is performed based on a first-fit algorithm of searching first unoccupied consecutive frequency slots closest to a selected frequency slot. In this case, a frequency slot dependency map is created based on the allocation of a plurality of optical signals to a plurality of frequency slots. The frequency slot dependency map relates groups including one or more frequency slots allocated to different optical signals interdependently.
If an optical signal departure event that an optical signal is dropped from an optical network occurs, a frequency slot occupied by the optical signal is released as a result. The optical signal departure event and the release of frequency slots cause fragmentation of the optical spectrum of the optical network.
In the method for eliminating fragmentation of optical spectrum described in Patent Literature 1, the fragmentation of the optical spectrum is eliminated by reallocating optical signals to different frequency slots based on the frequency slot dependency map. That is to say, by using the frequency slot dependency map after an optical signal departure event, frequency slots of one or more optical signals depending on a frequency slot of a dropped optical signal are determined. Based on that information, an optical signal is reallocated to a frequency slot released by the departure of the dropped optical signal (defragmentation).